xtensa: noMMU: allow handling protection faults

Many xtensa CPU cores without full MMU still have memory protection
features capable of raising exceptions for invalid instruction
fetches/data access. Allow handling such exceptions. This improves
behavior of processes that pass invalid memory pointers to syscalls in
noMMU configs: in case of exception the kernel instead of killing the
process is now able to return -EINVAL from a syscall.

Introduce CONFIG_PFAULT that controls whether protection fault code is
enabled and register handlers for common memory protection exceptions
when it is enabled.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

@@ -79,6 +79,7 @@ config STACKTRACE_SUPPORT
 
 config MMU
 	def_bool n
+	select PFAULT
 
 config HAVE_XTENSA_GPIO32
 	def_bool n
@@ -178,6 +179,16 @@ config XTENSA_FAKE_NMI
 
 	  If unsure, say N.
 
+config PFAULT
+	bool "Handle protection faults" if EXPERT && !MMU
+	default y
+	help
+	  Handle protection faults. MMU configurations must enable it.
+	  noMMU configurations may disable it if used memory map never
+	  generates protection faults or faults are always fatal.
+
+	  If unsure, say Y.
+
 config XTENSA_UNALIGNED_USER
 	bool "Unaligned memory access in user space"
 	help

arch/xtensa/kernel/traps.c

@@ -112,19 +112,19 @@ static dispatch_init_table_t __initdata dispatch_init_table[] = {
 #ifdef CONFIG_MMU
 { EXCCAUSE_ITLB_MISS,			0,	   do_page_fault },
 { EXCCAUSE_ITLB_MISS,			USER|KRNL, fast_second_level_miss},
+{ EXCCAUSE_DTLB_MISS,			USER|KRNL, fast_second_level_miss},
+{ EXCCAUSE_DTLB_MISS,			0,	   do_page_fault },
+{ EXCCAUSE_STORE_CACHE_ATTRIBUTE,	USER|KRNL, fast_store_prohibited },
+#endif /* CONFIG_MMU */
+#ifdef CONFIG_PFAULT
 { EXCCAUSE_ITLB_MULTIHIT,		0,	   do_multihit },
 { EXCCAUSE_ITLB_PRIVILEGE,		0,	   do_page_fault },
-/* EXCCAUSE_SIZE_RESTRICTION unhandled */
 { EXCCAUSE_FETCH_CACHE_ATTRIBUTE,	0,	   do_page_fault },
-{ EXCCAUSE_DTLB_MISS,		USER|KRNL, fast_second_level_miss},
-{ EXCCAUSE_DTLB_MISS,		0,	   do_page_fault },
 { EXCCAUSE_DTLB_MULTIHIT,		0,	   do_multihit },
 { EXCCAUSE_DTLB_PRIVILEGE,		0,	   do_page_fault },
-/* EXCCAUSE_DTLB_SIZE_RESTRICTION unhandled */
-{ EXCCAUSE_STORE_CACHE_ATTRIBUTE,	USER|KRNL, fast_store_prohibited },
 { EXCCAUSE_STORE_CACHE_ATTRIBUTE,	0,	   do_page_fault },
 { EXCCAUSE_LOAD_CACHE_ATTRIBUTE,	0,	   do_page_fault },
-#endif /* CONFIG_MMU */
+#endif
 /* XCCHAL_EXCCAUSE_FLOATING_POINT unhandled */
 #if XTENSA_HAVE_COPROCESSOR(0)
 COPROCESSOR(0),

arch/xtensa/mm/Makefile

@@ -4,7 +4,8 @@
 #
 
 obj-y			:= init.o misc.o
-obj-$(CONFIG_MMU)	+= cache.o fault.o ioremap.o mmu.o tlb.o
+obj-$(CONFIG_PFAULT)	+= fault.o
+obj-$(CONFIG_MMU)	+= cache.o ioremap.o mmu.o tlb.o
 obj-$(CONFIG_HIGHMEM)	+= highmem.o
 obj-$(CONFIG_KASAN)	+= kasan_init.o
 

arch/xtensa/mm/fault.c

@@ -25,6 +25,7 @@ void bad_page_fault(struct pt_regs*, unsigned long, int);
 
 static void vmalloc_fault(struct pt_regs *regs, unsigned int address)
 {
+#ifdef CONFIG_MMU
 	/* Synchronize this task's top level page-table
 	 * with the 'reference' page table.
 	 */
@@ -71,6 +72,9 @@ static void vmalloc_fault(struct pt_regs *regs, unsigned int address)
 
 bad_page_fault:
 	bad_page_fault(regs, address, SIGKILL);
+#else
+	WARN_ONCE(1, "%s in noMMU configuration\n", __func__);
+#endif
 }
 /*
  * This routine handles page faults.  It determines the address,